GaN layers with low dislocation density have been fabricated be means of facet-controlled epitaxial lateral overgrowth (FACELO) via low-pressure metalorganic vapor phase epitaxy (LP-MOVPE). The distribution of the dislocations in FACELO GaN was inspected by observation of InGaN growth pits. For FACELO with {11-20} facets as the first step, the dislocations concentrate only in the window region. For FACELO with {11-22} facets as the first step, the dislocations exist only in the coalescence region. The double FACELO, which was FACELO with {11-20} on FACELO with {11-22}, was demonstrated and dislocation density of less than 105 cm−2 was achieved.

We consider the behavior of the absorption coefficient and luminescence spectrum in the steady state when III-nitrides semiconductors (compounds GaN, AlN, and InN) are in far-fromequilibrium conditions created by an electric field. We analyze the high frequency part of the spectra obtaining a generalization of the Roosbroeck-Schockley relation, δRS(ω, EF), the ratio between the frequency dependent luminescence I(ω) and the absorption coefficient α(ω), for nonequilibrium conditions which are dependent on the electric field intensity EF. We show that the carrier's temperature within a small error is proportional to d ln[δRS(ω, EF)]/dω.

The performance characteristics are reported for continuous-wave (cw) InGaN multiple-quantum-well laser diodes grown on epitaxially laterally overgrown GaN on sapphire substrates by metalorganic chemical vapor deposition. Room-temperature cw threshold currents as low as 41mA with operating voltages of 6.0V were obtained. The emission wavelength was near 400 nm with output powers greater than 20 mW per facet. Under cw conditions laser oscillation was observed up to 90°C. A significant reduction in thermal resistance was observed for laser diodes transferred from sapphire onto Cu substrates by excimer laser lift-off, resulting in increased cw output power of more than 100mW.

We have measured high spatial/depth resolution (2-3 [.proportional]m) thermal conductivity (κk) at 300K before and after plasma-induced effects on a series of n-GaN/sapphire (0001) samples fabricated by hydride vapor phase epitaxy (HVPE) using a ThermoMicroscope'as scanning thermal microscope (SThM). The sample thicknesses were 50 ± 5 [.proportional]m and the carrier concentrations ~ 8 × 1016 cm-3, as determined by Hall effect measurements. The thermal conductivity before treatment was found to be in the 1.70 – 1.75 W/cm-K range, similar to that previously reported for HVPE material with this carrier concentration and thickness [D. I. Florescu et al., J. Appl. Phys. 88, 3295 (2000)]. The samples were processed under constant Ar gas flow and pressure fora fixed period of time (5 min). The only variable processing parameter was the DC bias voltage (125 – 500 V). After the initial 125 V procedure κ exhibited a decrease linear in the DC voltage in the investigated range. At 125 V the thermal conductivity was only slightly less (κ ~ 1.65 W/cm-K) than the untreated case. κ had dropped to ~ 0.3 W/cm-K for the 500 V situation. The implications of these results for device applications in the area of high power opto-electronics and high power electronics will be discussed.

The growth polar direction during metalorganic chemical vapor phase epitaxy of wurtzite GaN films was shown to affect the optical properties in terms of impurity and vacancy-type defect incorporation during the growth. The GaN film grown towards the Ga- face (0001) (+c polarity) exhibited clear excitonic features in its optical absorption and luminescence spectra up to room temperature. Conversely, the film with the N-face (000-1) (-c polarity) exhibited a broad emission band, which is located in the broadened absorption tail. The Stokes shift remained even at 300 K. The difference between the two was explained in terms of the presence of impurity-induced band tail states in –c GaN due to increased impurity density and enhanced incorporation of large volume vacancy-type defects, which were confirmed by secondary ion mass spectrometry [Sumiya et al., Appl. Phys. Lett. 76, 2098 (2000)] and monoenergetic slow positron annihilation technique.

We have fabricated GaN quantum dots (QDs) in AlN confined layer structures by molecular beam epitaxy. The size distribution and density of the QDs have been estimated from an atomic force microscopy study. Very high quantum efficiency of photoluminescence (PL) has been obtained in some samples with QDs. Compared to the GaN bulk samples, it increased by orders of magnitude. In some samples the quantum size effect dominated, resulting in the blue-shift of the QD related PL peak, whereas in the samples with larger dots a red-shift up to 0.8 eV has been observed, which is related to strong polarization effects. We have observed a blue-shift of the PL peak with excitation intensity in the samples with large dots due to screening effect. The temperature-induced quenching of PL occurs at higher temperatures compared to bulk GaN due to the confinement of nonequilibrium carriers in the QDs. An excited state has been observed in some samples.

We have used an ultrahigh vacuum scanning electron microscope to carry out cross sectional secondary electron imaging, cathodoluminescence spectroscopy, and cathodoluminescence imaging on GaN grown on sapphire by hydride vapor phase epitaxy. These measurements provide evidence for deep level defects highly localized at the GaN, sapphire interface as well as defects extending into both the semiconductor film and the substrate. The different spatial distributions of these radiative defects provide information on the physical origin of these electrically active features.

Defect related photoluminescence (PL) in unintentionally doped GaN layers grown by molecular beam epitaxy (MBE) was studied. Under certain growth conditions, we observed new defect-related bands: a red band with a maximum at about 1.88 eV and a green band with a maximum at about 2.37 eV. The quenching of these bands with increasing temperature took place with an activation energy of about 120-140 meV at temperatures above 100 K. Moreover, the red band exhibited an increase of PL intensity with an activation energy of 1.2 meV in the range of 10-60 K. The observed behavior is explained by invoking a configuration coordinate model and that we speculate the defects to be partially nonradiative and related to Ga atoms.

We present the results of the high magnetic field studies of properties of two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures grown over high-pressure bulk GaN, sapphire, and insulating SiC substrates. The experimental results include the low field Hall measurements, cyclotron resonance measurements, and cryogenic temperature Quantum Hall Effect studies as well as room-temperature characteristics of High Electron Mobility Transistors fabricated on all these substrates. The room temperature high field measurements allow us to clearly separate the contributions of a parasitic parallel conduction from 2DEG conduction in all investigated heterostructures.

The magnetotransport measurements are performed in the magnetic fields up to 30 Tesla for temperatures between 50mK-300K. This high magnetic field in combination with very high mobilities (over 60.000 cm2/Vs) in the sample on the bulk GaN substrates allow us to observe features related both to cyclotron resonance and spin splitting. The temperature dependence of this splitting determines the spin and cyclotron resonance energy gaps and, in combination with cyclotron resonance and tilted field experiments, allows us to determine the complete energy structure of 2DEG conduction band. We also present the first experimental results showing so called “the exchange enhancement” of the energy gaps between spin Landau levels.

We have developed a new method to prepare low-dislocation-density GaN by using periodically grooved substrates in a conventional MOVPE growth technique. This new approach was demonstrated for GaN grown on periodically grooved α-Al2O3(0001), 6H-SiC(0001)Si and Si(111) substrates. Dislocation densities were 2×107 cm−2 in low-dislocation-density area.

Deep level defects in oxygen doped GaN grown by metal-organic vapor phase epitaxy were investigated. Using steady-state photocapacitance (SSPC) spectroscopy, three deep levels with optical ionization energies of 1.0, 1.4, and 3.25 eV were observed in both nominally undoped and oxygen-doped samples. The total deep level defect concentrations ranged from 6 × 1015 cm-3 in undoped films to 3 × 1016 cm-3 in oxygen-doped films. The concentration of the 3.25 eV level defect increased upon oxygen doping, while the concentrationof the 1.0 and 1.4 eV levels were essentially dopant independent. From the measured concentrations the formation energies of the defects were calculated and compared to energies calculated using density functional theory.

Two kinds of InGaN/GaN single quantum well with emission wavelength of 500nm were obtained by employing piezoelectric effect and phase separation with different growth methods. Their emission properties are studied by measuring temperature and photo-excitation power dependent photoluminescence and electrolluminescence. The 500nm luminescence from the sample obtained by piezoelectric effect shows very weak temperature and excitation power dependent, on the other hand, it shows profound band tail broadening at both high temperature and low excitation power for phase separated sample which mean the strong localized state exist. We observed highly bright spots with longer wavelength then that of matrix in the phase separation sample and we believe that high emission efficiency can be obtained from these indium rich clusters.

We report on the effect of thermal annealing on the photoluminescence properties of a Ga0.65In0.35N0.02As0.98/GaAs single quantum well. Thermal annealing is shown to decrease the strong nitrogen-induced localization effects observed at low temperatures and to reduce the full width at half maximum of the emission peak. It also induces a strong blue shift of the emission peak energy, which is thought not to arise from an In-Ga interdiffusion alone, as it is much larger than in a nitrogen-free reference single quantum well.

Low temperature photoluminescence is used to study the levels introduced by low-dose implantation of Mg, C, and Be into GaN, together with Ne, Al, P, or Ar co-implantation species. None of the co-implants successfully creates shallow C or Be acceptors. The yellow (2.2 eV) PL band is strongly introduced by both C and Be implants, but not by Ar, Al, P, or Mg. We propose that it involves Ga vacancies stabilized by complexing with C and Be interstitial (or CGa) donors, which explains why C and Be are absent as substitutional acceptors. We observe excitons bound to isoelectronic PN in P-implanted GaN. They are absent in P+Mg implanted GaN, in which we observe new donor-to-acceptor pair peaks due to two deeper donor levels. We assign these levels to PGa antisite double donors, which are stable in p-type material

Transmission electron microscopy has been used to examine dislocations present in an epitaxial laterally overgrown (ELOG) sample of GaN grown on (0001)sapphire. Studies of both plan-view and cross-sectional samples revealed arrays of dislocations present in the (11-20) boundary between the seed and the wing (overgrown) material and at the meeting front between adjacent wings, as well as dislocations in the form of half-loops extending into the wing regions. Both the boundary and half-loop dislocations had 1/3<11-20> Burgers vectors which were either perpendicular (boundary dislocations) or at 30°s (half-loops) to the boundary plane. Large angle convergent beam electron diffraction was used to show that the boundary dislocations and halfloops correlated respectively with tilts and twists of the wing material about (11-20). A model is proposed whereby the half-loops are generated from threading dislocations by shear stresses acting along the stripe direction. The origin, and elimination, of these stresses is discussed.

Electron transport properties in the Al0.15Ga0.85N/GaN heterostructure field effect transistors (HFETs) have been examined from room temperature up to 400°C. The temperature dependencies of the two-dimensional electron gas (2DEG) mobility have been systematically measured for the samples with different 2DEG densities. The 2DEG mobility has decreased with increasing the temperature, however, its decrease ratio has been no longer large above 300°C. Moreover, the 2DEG mobility has found to be less dependent on the 2DEG density at higher temperatures. These observed features indicate that the 2DEG mobility above room temperature is limited by longitudinal optical (LO) phonon scattering, as is expected by theoretical prediction. The observed 2DEG mobilities at 400°C were as high as from 100 to 120 cm2/Vs, directly providing the evidence for suitability of the HFET of this material system for high-temperature applications. The temperature dependence of the transconductance (gm) of a HFET device has also been examined up to 400°C. It has been revealed that the temperature dependence of gm has basically the same features as those of the 2DEG mobility in the corresponding temperature region.

The surface morphologies of homoepitaxial GaN films grown by molecular beam epitaxy (MBE) on metalorganic chemical vapor deposition (MOCVD) grown GaN template layers were investigated, using atomic force microscopy (AFM). Typical surface morphology of MBE-grown films on MOCVD-templates was dominated by spiral hillocks due to the high density of dislocations having a screw character and large driving force of MBE growth. Introduction of the AlN multiple interlayer (AlN -MIL) into MBE-GaN layers suppressed the formation of spiral hillocks. It was attributed to obstructing the dislocation propagation by AlN-MIL. Migration enhanced epitaxy (MEE) growth of GaN also reduced the density and tightness of spiral hillocks. This observation was attributed to that MEE growth technique decreased the driving force of growth.

We have studied an influence of pressure on the emission and absorption spectra measured from various types of InGaN structures such as epilayers, quantum wells and quantum dots. While the known pressure coefficients of GaN and InN bandgaps are in the range 40-25 meV/GPa, the experimental observation for the light emission shift with pressure for InGaN alloys is dramatically different. With the increasing In content and thus decreasing emission energy the observed pressure coefficients become very small eventually reaching zero or even slightly negative values! We have observed a much weaker trend for the decrease of the pressure coefficient for the absorption edges of InGaN. First principle calculations of InGaN band structure and its modification with a pressure are not able to explain the huge effect observed in the emission experiment but are in a good agreement with the results obtained in optical absorption measurements. We discuss here the possible mechanisms which can account for extremely low pressure coefficient of the light emission and the discrepancy between sensitivity light emission and absorption on applied pressure in InGaN alloys.

We use electron holography to profile the local internal potential due to spontaneous polarization and piezoelectric effects in strained quantum well structures of wurtzitic group III nitrides. Profiles of the electrostatic potential across a GaN/InxGa1−xN/GaN quantum well structure show the existence of internal electric fields of about –2.2 ± 0.6 MV/cm, and a potential drop across the quantum well of 0.6 ± 0.16 V. The electric fields indicate an average indium composition of 15% in the quantum well, for a thickness of 2.7 nm. This indium composition compares well with measurements by energy-disperse spectroscopy of 18 ± 2 %. Screening effect is not observed under these experimental conditions.

Aluminum Nitride (AlN) is a promising piezoelectric material for Acoustic Wave (AW) sensor application due to its high acoustic velocity, linear thermal coefficient and high electromechanical coupling coefficient. Epitaxial AlN thin films were successfully grown on the Sapphire C plane substrate by Plasma Source Molecular Beam Epitaxy (PSMBE). Standard two-port resonator structures were fabricated on the AlN/Sapphire by standard photolithography. Excimer laser micromachining techniques were utilized to fabricate microgroove gratings on the surface of thin film. Beside Surface Acoustic Wave (SAW), Surface Transverse Wave (STW) propagation was also found on the devices with those microgroove and classical metal strip gratings. Laser micromachinined microgroove gratings were found to enhance the propagation of STW. Further, sensors based on STW showed little attenuation in the liquid environment, while sensors based on SAW suffered excess loss when exposed to liquid.